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Introduction
Volatile Organic Compounds (VOCs) have become a major environmental focus as global industries work toward cleaner, more sustainable production. These compounds, widely found in chemical processing, metallurgy, coating, printing, and many other industrial sectors, pose significant risks to air quality, human health, and ecological stability. As environmental regulations tighten worldwide, industries must adopt efficient VOC control systems to reduce emissions and ensure compliance.
This article provides an in-depth overview of how VOCs are generated across major industries and explores the most effective technologies for their treatment. From coal chemical processing to printing and packaging, understanding these mechanisms is essential for designing solutions that are both environmentally sound and economically feasible.
What Are VOCs?
Volatile Organic Compounds are a broad class of carbon-based chemicals with high vapor pressure at room temperature, allowing them to easily evaporate into the air. VOCs typically include substances with a normal-pressure boiling point between 50°C and 260°C or those with a saturated vapor pressure above 133.32 Pa at ambient conditions.
Common VOC Categories
Based on chemical structure, VOCs fall into eight major groups:
Alkanes
Aromatic hydrocarbons
Alkenes
Halogenated hydrocarbons
Esters
Aldehydes
Ketones
Other organic compounds
Typical VOC Examples
Aromatic hydrocarbons: benzene, toluene, xylene, styrene
Chain hydrocarbons: butane, gasoline components
Halogenated hydrocarbons: carbon tetrachloride, chloroform
Alcohols and aldehydes: methanol, acetaldehyde, acetone
Esters: ethyl acetate, butyl acetate
Others: acetonitrile, acrylonitrile, chlorofluorocarbons
These compounds originate from fuel combustion, chemical reactions, solvent evaporation, and various industrial processes. Due to their high reactivity and toxicity, VOCs require systematic control.
Major Industrial Sources of VOC Emissions
1. VOCs in the Coal Chemical Industry
The coal chemical sector is one of the most significant industrial contributors to VOC emissions. VOCs primarily originate from two processes:
Coal coking
Coal gasification to synthetic gas
1.1 VOC Emissions During Coal Coking
Coking involves heating coal at high temperatures, causing complex organic compounds to volatilize. Emissions mainly occur in two phases:
A. Coal Charging Stage
When raw coal is loaded into high-temperature coke ovens, it encounters hot surfaces and releases a mixture of:
Polycyclic aromatic hydrocarbons
Tar vapor
Organic gases
These pollutants contribute to occupational hazards and environmental contamination.
B. Coking Byproduct Recovery Area
Key areas include the condensation unit, desulfurization unit, ammonium sulfate unit, and crude benzene unit. Each generates different VOC profiles:
Condensation Section
Emissions: ammonia, hydrogen sulfide, naphthalene, mixed VOCs
Sources: tar tanks, ammonia water tanks, pipelines, water seals
Characteristics: high concentration, large fluctuations, moisture-rich gas
Desulfurization & Ammonium Sulfate Section
Emissions: sulfur-containing gases, ammonia, small VOC content
Continuous emissions with high ammonia concentrations
Crude Benzene Section
Emissions: benzene, toluene, xylene
Gas volume small but concentration very high
Wastewater Treatment Area
Emissions: benzene, phenols, sulfides, nitrogen organic compounds
Comes from equalization tanks, accident tanks, anaerobic tanks, sludge treatment
This combination makes treatment challenging due to its complex composition.
1.2 VOCs in Coal Gasification and Natural Gas Production
Coal gasification plants produce VOC-laden tail gases during:
Low-temperature methanol wash
Gas/liquid storage tanks (breathing losses)
Wastewater treatment
Oil storage units
A. Low-Temperature Methanol Wash Tail Gas
This stream includes:
Methane
Ethylene, ethane
Propane, propylene
Methanol vapor
It is difficult to reuse and typically treated with RTO (Regenerative Thermal Oxidizer) for complete incineration.
Why RTO instead of RCO?
RCO catalysts are vulnerable to sulfur poisoning and have limited regeneration, making RTO more robust for coal chemical applications.
B. Storage Tank Breathing Losses
Gas/liquid storage tanks release vapors containing sulfur compounds, ammonia, and VOCs during temperature and pressure changes. These gases also require thermal oxidation.
C. Wastewater Treatment VOCs
These emissions mainly arise from:
Preliminary treatment (oil separation, equalization, acidification)
Aeration tanks
Sludge dewatering rooms
The concentrations vary dramatically, and moisture content is high.